22 Geobiology of the Anthropocene

22 GEOBIOLOGY OF THE ANTHROPOCENE Daniel P. Schrag Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138, USA 22.1 Introduction 22.2 The Anthropocene Homo sapiens first appeared on the Earth somewhere The adoption of the term ‘Anthropocene’ is commonly in Africa roughly 200 000 years ago. It happened with credited to Paul Crutzen, the Nobel-prize winning little fanfare; few could have imagined that this new chemist, in a speech in 2000, although the recognition of species of primate would someday disrupt the Earth human impact on the Earth and the declaration of system to the point of defining a new geologic epoch a new geological epoch long precedes Crutzen around its legacy. Indeed, the first 150 000 years of (Zalasiewicz et al., 2011). Already in 1871, Italian geolo- the natural history of our species, mostly in Africa, gist Antonio Stoppani used the term ‘Anthropozoic’ to were fairly uneventful for reasons still not well describe ‘new telluric force, which in power and uni- understood. But then, with migration out of Africa, versality may be compared to the greater forces of things started to change. Having mastered new hunt- earth.’ Joseph LeConte, in his Elements of Geology (1878) ing skills, humans began to perturb their ecosystems, uses the term ‘Psychozoic’ to describe the age of man, first by overhunting large animals, which also characterized by the ‘reign of mind.’ Even Charles Lyell deprived rival predators of adequate food supplies. pondered the enormous impact that humans were hav- Then with the development of agriculture in the last ing on the Earth, for he recognized that some might see 10 000 years, humans began an appropriation of the in it a challenge to his arguments about the uniformity Earth’s surface for food, fuel and fiber that continues of nature’s laws. In the original version of his Principles to this day. More recently, the industrial revolution, of Geology (1830), Lyell discusses the modern origin of spurred on with cheap, abundant energy from fossil humans, and the question of organic carbon, made humans major players in Earth’s geochemical cycles, including nitrogen and ‘whether the recent origin of man lends any support to the carbon. The latter now threatens to end the Pleistocene same doctrine, or how far the infl uence of man may be glacial cycles and return the Earth to a state not seen considered as such a deviation from the analogy of the for 35 million years. The future of human interactions order of things previously established, as to weaken our with the Earth system remains uncertain, but the confi dence in the uniformity of the course of nature.’ impact of human actions already taken will last for more than 100 000 years. Avoiding massive Lyell states his concern clearly: disruptions to geobiological systems in the future is likely to require, ironically, even larger interventions ‘Is not the interference of the human species, it may be by humans through advanced technology, the final asked, such a deviation from the antecedent course of step in a transition to the engineered epoch of Earth physical events, that the knowledge of such a fact tends to history. destroy all our confi dence in the uniformity of the order of Fundamentals of Geobiology, First Edition. Edited by Andrew H. Knoll, Donald E. Canfield and Kurt O. Konhauser. © 2012 Blackwell Publishing Ltd. Published 2012 by Blackwell Publishing Ltd. 425 KKnoll_c22.inddnoll_c22.indd 442525 22/16/2012/16/2012 77:49:51:49:51 PPMM 426 Fundamentals of Geobiology nature, both in regard to time past and future? If such an and the rise in greenhouse gases observed in ice cores innovation could take place after the earth had been exclu- (Crutzen and Stoermer, 2000). A different view comes sively inhabited for thousands of ages by inferior animals, from William Ruddiman, who argued that human inter- why should not other changes as extraordinary and ference in the climate system began 7000 years ago with unprecedented happen from time to time? If one new the development of agriculture. Ruddiman pointed to cause was permitted to supervene, differing in kind and the concentration of atmospheric CO2, which was energy from any before in operation, why may not others 260 ppm approximately 8000 years ago, and suggested have come into action at different epochs? Or what secu- that it should have fallen by 20 ppm, synchronous with rity have we that they may not arise hereafter? If such be changes in the Earth’s orbit around the sun, as it had the case, how can the experience of one period, even during previous interglacials Intervals. Instead, atmos- though we are acquainted with all the possible effects of pheric CO2 rose to 280 ppm prior to the industrial revo- the then existing causes, be a standard to which we can lution, a net difference of 40 ppm that Ruddiman refer all natural phenomena of other periods?’ attributed to the release of carbon dioxide from deforestation. It is this reversal of greenhouse gases, In the early 19th century, Lyell already recognized the Ruddiman claimed, that stabilized the climate of the magnitude of human impact on the Earth system. Holocene and allowed human civilizations to flourish (Ruddiman, 2003, 2007). ‘When a powerful European colony lands on the shores Examination of the carbon cycle does not support of Australia, and introduces at once those arts which it Ruddiman’s hypothesis. Over thousands of years, most has required many centuries to mature; when it imports of the carbon released from deforestation would dis- a multitude of plants and large animals from the oppo- solve in the ocean, which means that a net change in site extremity of the earth, and begins rapidly to extirpate atmospheric CO2 of 40 ppm would require roughly 600 many of the indigenous species, a mightier revolution is billion tonnes of carbon to be released from the land, an effected in a brief period, than the fi rst entrance of a sav- amount equivalent to the entire modern terrestrial bio- age horde, or their continued occupation of the country sphere. Moreover, such a large release of carbon from for many centuries, can possibly be imagined to have biomass would change the isotopic composition of car- produced.’ bon reservoirs, as recorded in shells and ice cores; no such change is observed. Finally, the rise in atmospheric One can only imagine what Lyell would think were he CO2 has been linear over the last 7000 years, but the to see the scale of human activities today. expansion of agriculture was not. It seems most likely Lyell’s argument was that humans – even with that early agriculture had a smaller impact on the carbon their disruptions to natural ecosystems, even cycle than Ruddiman claimed. with their morality and their unique ability to inter- But that is not to say that early humans had little effect pret nature through an understanding of its natural on their environment. It is quite clear that early humans laws – remain bound by those laws. This is what changed their ecosystems by overhunting of large ani- allowed Lyell to preserve his uniformitarian theory, mals long before the invention of agriculture (Alroy, which he viewed as essential for understanding Earth 2001). The extinction of larger mammals and birds in history. But there are closely related questions look- Asia, Europe, Australia, the Americas, and New Zealand ing to the future that Lyell did not address. Is the and the Pacific Islands immediately followed the spread Anthropocene (to use the modern form) recognizable of human populations across these regions. The timing among other geological epochs? When did it begin of the extinctions is diachronous, as predicted by the and when will it end? And what, among all the many over-hunting hypothesis; moreover, the disappearance features of the geobiological record of the of large animals is a distinctive feature of the extinction Anthropocene, will be most recognizable millions of across every climatic regime, from the tropics to the tem- years in the future? In this chapter, I describe some perate zones, and in both hemispheres, refuting a aspects of the geobiology of the Anthropocene in an climatic explanation for the extinction as some have attempt to address these questions. proposed. Only in sub-Saharan Africa did megafauna survive the rise of human society, and the reason for their persistence remains a mystery. 22.3 When did the Anthropocene begin? In Guns, Germs and Steel (Diamond, 1997), Jared In a 2000 newsletter of the International Geosphere- Diamond proposed that the co-evolution of African Biosphere Program, Crutzen and Stoermer suggested megafauna with humans, as well as with earlier homi- that the transition from the Holocene to the Anthropocene nids, allowed large mammals to survive – the essential began near the end of the 18th century, coincident with claim is that large animals in Africa learned to be afraid the invention of the steam engine by James Watt (1784) of humans, a behaviour honed by natural selection. This KKnoll_c22.inddnoll_c22.indd 442626 22/16/2012/16/2012 77:49:51:49:51 PPMM Geobiology of the Anthropocene 427 hypothesis predicts good news for future conservation 1930, 2 billion; and, in 1974, human population efforts as it suggests that African megafauna have an reached 4 billion. It sits now very close to 7 billion, instinctive key to their own survival – i.e., their fear of and most demographic projections predict a peak humans. An alternative explanation, however, allows around 9 billion sometime in the middle of the 21st less optimism. It seems possible that large mammals in century. How Earth’s ecosystems will fare on a planet Africa survived not because of co-evolution with with 9 billion human beings is a question we will humans, but because human occupation of sub-Saharan address here.

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